provider: New module used to expose extension points for core services

Introducing the concept of providers: co-processes that talk with the editor
through the remote API and provide implementation for one or more core
services.

The `provider_register` function and it's API wrapper can be used by channels
that want to self-register as a service provider.

Some old builtin vim features will be re-implemented as providers. The
`provider_has_feature` function is used to check if a provider
implementing a certain feature is available(It will be called by the `has`
vimscript function to check for features in a vim-compatible way)

This implements the provider module without exposing any extension points, which
will be done in future commits.

src/nvim/eval.c

@@ -85,6 +85,7 @@
 #include "nvim/api/private/helpers.h"
 #include "nvim/os/msgpack_rpc_helpers.h"
 #include "nvim/os/dl.h"
+#include "nvim/os/provider.h"
 
 #define DICT_MAXNEST 100        /* maximum nesting of lists and dicts */
 
@@ -9807,6 +9808,10 @@ static void f_has(typval_T *argvars, typval_T *rettv)
     }
   }
 
+  if (n == FALSE && provider_has_feature((char *)name)) {
+    n = TRUE;
+  }
+
   rettv->vval.v_number = n;
 }